I've owned SR60's, SR225's, and SR325is's. I've listened to SR80's and SR125's.
I'll address your question first. I've never heard the older 325 model. My only experience with the 325 line comes from the 325is model, which seems to have different characteristics than the older version. The drivers in the Prestige line are indeed the same. I once tracked down the company in Mexico that produces them, but, my memory isn't what it was 5 minutes ago. As you go up the line in price, the drivers are supposedly more closely matched. Additionally, different models have different amounts of dampening material on the rear of the driver as well as the rear of the driver housing. I don't know what those differences are from model to model, nor do I care to find out with my cans as Guinea Pigs. The main cable on the 325is line is thick enough to whip a horse with, but the secondary cabling is as thin as that of the SR60 model.
When you say "better", are you talking head-fi esoterica or are you talking about leaning back in a recliner and dwelling on heavy rifts? Also, before I answer, I'm sure you've discovered that above all other variables, Grados are genre-specific: I feel they're best suited for those who've got a rock and roll heart. As I'm also sure you've discovered, most headphone manufacturers have a house sound. Grados aren't for those who seek massive a sound stage and dimensional presence. They're punchy, revealing, and raw. They make guitars do things that they have no business doing. That's a compromise I make when I put either of my Grado cans on.
The difference is that the 325's are just "more". It's difficult to explain without hearing. I may be describing what people describe as "brightness". Drums are punchier, guitars scream with more life, pianos jump out at you, winds have more life, again, it's nothing I can put in to words. It's just, more. With that said, the SR60's, being "less" are a better (albeit not good) choice for a wider range of genres. Something tells me that is what those folks are addressing as "better".
I didn't actually answer your question with anything tangible, for that I'm sorry. I'd really prefer you hear the difference.
I can even meet you at the shop on a weeknight after 9. There's enough big silver there to provide more than proper amplification for any and all Grado cans.
